Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the package for liquid or pourable food products known as Tetra Rex Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
According to a known forming technique, the packaging material is cut into blanks. The blanks are firstly erected to form respective open sleeves, which are sealed at their bottom ends. Afterwards, the open sleeves are filled with the pourable product through their open top ends, and top ends are then sealed, so as to complete the formation of packages.
It is also known to apply an opening device onto a respective open sleeve formed according to the previously described technique, before the filling of the sleeve with the pourable product.
In particular, the opening device comprises:                a flange to be sealed onto the packaging material of the sleeve in an area surrounding a hole thereof;        a neck, which protrudes from the flange; and        a cap, which is screwed onto the neck.        
In order to apply each opening device onto the relative sleeve, it is known to use a sealing system comprising:                an anvil; and        a sonotrode.        
The anvil is movable, through the open top end of the sleeve, between:                a rest position, in which it is arranged outside the sleeve; and        an operative position, in which it is arranged inside the sleeve.        
The sonotrode substantially comprises;                a head, which defines a sealing surface of the sonotrode, which receives the neck and the cap of the opening device; and        one or more drive unit, which comprises a stack of alternate piezoelectric ceramic plates and conductive metal sheets.        
The drive unit is coupled to an alternate current generator. In this way, the piezoelectric ceramic plates convert the electrical supplied voltage into a mechanical strain, which causes the oscillation of the head and, therefore, the sealing of the opening device onto the packaging material of the package.
Furthermore, sonotrode is movable, through a hole of the package, between:                a rest position, in which it is arranged outside the sleeve; and        an operative position, in which the opening device engages the hole of the sleeve.        
The movement of the sonotrode and the anvil is synchronized with the movement of the open sleeves.
In particular, when both the sonotrode and the anvil are in the respective operative positions, the head of the sonotrode is pressed against the anvil, so as to keep the opening device in a fixed sealing position with respect to the packaging material of the sleeve surrounding the hole.
At this stage, the generator is activated, so as to seal the opening device to the packaging material of the sleeve surrounding the hole.
The Applicant has found that the correct positioning of the opening device inside the sonotrode is very critical for the efficient sealing of the opening device to the respective open sleeve.
In greater detail, the Applicant has found that in case the opening device is not properly positioned inside the sonotrode, there is the risk that the ultrasonic vibration is transferred to the cap of the opening device, instead of to the flange, thus penalizing the sealing of opening devices.
A need is felt within the industry to ensure a proper positioning of the opening device inside the sonotrode.